The short answer is they don’t!
This is because, unfortunately, minors have very few legal rights and deciding one’s own custody is never one of them; only the judge gets to decide custody. However, a child’s meaningful preference is one of nine “best interest” factors which the judge can consider in deciding physical custody.
When is a child old enough for their preference to be considered meaningful? Again, there is no specific age at which a “preference” is considered to be “meaningful”; it depends on the child’s level of maturity at the time the preference is expressed. A 14 year old’s preference to live with Mom because she “has no rules” is unlikely to be considered at all, while a 12 year old’s mature, reasoned preference will likely be given weight.
As one judge put it: “A relatively mature teenager’s reasoned preference is not so lightly disregarded.” Be careful, however, because a teenager’s preference is not reliable if it is “manipulated or unduly influenced by a parent.”